1. Field of the Invention
The present invention relates to a motor control apparatus, and more particularly to a motor control apparatus having a function for protecting LCL filter.
2. Description of the Related Art
In a motor control apparatus which drives a motor of a working machine, a forging machine, an injection molding machine, an industrial machine, a robot, or the like, a rectifier which converts alternating current (AC) power of a three-phase AC power supply to direct current (DC) power and an inverter which converts the DC power that has been output by the rectifier to AC power for driving the motor are used.
Recently, a rectifier (PWM rectifiers) using pulse width modulation (PWM) has been more and more extensively employed due to demands for reducing harmonics in a power supply and a reactive power.
To perform a switching control by the PWM, the PWM rectifier as described above outputs to a path to the three-phase AC power supply a rectangular wave AC voltage including that of high frequency of several kilohertz or more. To make only a power supply frequency component out of the rectangular wave pass, a low-pass filter is commonly provided between the PWM rectifier and the three-phase AC power supply.
FIG. 1 is a configuration diagram of a conventional motor control apparatus. A conventional motor apparatus 1000 includes a PWM rectifier 110 which converts an AC power from a three-phase AC power supply 20 to a DC power and an inverter 180 which converts the DC power to an AC power for driving a motor 30. Further, an LCL filter 120 as a low-pass filter serially connecting a damping resistor 123 to a capacitor 124 and including an inductance unit A 121 and an inductance unit B 122 at one end of the damping resistor 123 is provided between the three-phase AC power supply 20 and the PWM rectifier 110. In addition, a cooling fan 130 for cooling each element of the LCL filter 120 is also usually provided.
The LCL filter 120 suppresses a ripple of a high frequency current flowing into from a PWM rectifier 110 side while reducing a volume and a cost of the LCL filter 120 so as to be usually configured commonly in such an asymmetric manner that an inductance of the inductance unit B 122 on the PWM rectifier 110 side is high and an inductance of the inductance unit A 121 on a three-phase AC power supply side is low.
Waveforms of a voltage and a current on each of the three-phase AC power supply 20 side and the PWM rectifier 110 side of the LCL filter 120 are illustrated in FIGS. 2A-2D. FIG. 2A is a graph illustrating a temporal change of a voltage on the three-phase AC power supply side, FIG. 2B is a graph illustrating a temporal change of a current on the three-phase AC power supply side, FIG. 2C is a graph illustrating a temporal change of a voltage on the PWM rectifier side, and FIG. 2D is a graph illustrating a temporal change of a current on the PWM rectifier side.
When a power line of the LCL filter 120 having an asymmetric configuration is connected in a manner reverse to that as illustrated in FIG. 1, in other words, the inductance unit A 121 having a low inductance is connected on the PWM rectifier 110 side and the inductance unit B 122 having a high inductance is connected on the three-phase AC power supply 20 side, the ripple of a high frequency current flowing into from the PWM rectifier 110 side fails to be sufficiently suppressed so that heat generation of the LCL filter 120 increases.
In the motor control apparatus as described above, a current waveform in a case of a normal connection of the power line of the LCL filter 120 having an asymmetric configuration is illustrated in FIG. 3A, and a current waveform in a case of a reverse connection is illustrated in FIG. 3B. As illustrated in FIG. 3B, when the power line of the LCL filter is reversely connected, a current in which a ripple fails to be suppressed is found out to flow. When such a current continues to flow, noises or abnormal heat generation in a core portion of the inductance unit A 121 originally designed on the assumption that a current containing no ripple flows may occur. Along with this, damage or abnormal heat generation in the damping resistor 123 and the capacitor 124 into each of which the ripple flows may occur as well.
Since a temperature sharply increases in this case, stopping an operation of the PWM rectifier as soon as possible is to be performed. Conventionally, in this regard, a temperature detection unit for detecting a temperature of an element in an LCL filter is provided to detect abnormality of the LCL filter (for example, Japanese Laid-open Patent Publication No. 2013-246683A). FIG. 4 is a configuration diagram of a motor control apparatus 2000 which is another example of the conventional motor control apparatus. As illustrated in FIG. 4, the LCL filter 120 serially connects the damping resistor 123 to the capacitor 124 and includes the inductance unit A 121 and the inductance unit B 122 at one end of the damping resistor 123. A temperature sensor 125 is disposed, for example, in the vicinity of the damping resistor 123, and outputs information on a detected temperature to a temperature detection unit 140. A temperature detection result detected by the temperature detection unit 140 is transmitted to a determination unit 150, and the determination unit 150 controls the PWM rectifier 110 on the basis of the temperature detection result. For example, a temperature detected by the temperature detection unit 140 exceeds a predetermined value (or a temperature at a normal time which has been stored), the operation of the PWM rectifier 110 is stopped, and protection of the LCL filter 120 is performed.
In such a manner, conventionally, temperature information of the LCL filter is obtained, abnormality is detected when a temperature is at a predetermined value or more, and the operation of the PWM rectifier is stopped.
Further, as illustrated in FIG. 4, the LCL filter 120 is also usually provided with the cooling fan 130. The temperature of the LCL filter 120 increases not only in the case of the reverse connection of the power line of the LCL filter but also due to a stop of the cooling fan 130, which may result in an abnormal temperature.
An increase in temperature due to the stop of the cooling fan 130 is greater as the cooling fan 130 is more highly effective. Further, an increase in temperature due to the reverse connection of the power line of the LCL filter 120 is greater as a ratio of an inductance magnitude of the inductance unit B 122 to that of the inductance unit A 121 is larger.
When reduction in a volume and a cost of the LCL filter 120 is considered, the cooling fan 130 is usually designed in such a manner as to have a minimum performance required. Meanwhile, the ratio of an inductance magnitude of the inductance unit B 122 to that of the inductance unit A 121 is usually designed in such a manner as to be as large as possible. As the result, a loss in the case of the reverse connection of the power line of the LCL filter 120 is greater than a loss in the case of the stop of the cooling fan 130. Consequently, the increase in temperature due to the reverse connection of the power line of the LCL filter 120 is greater than the increase in temperature due to the stop of the cooling fan 130.
Thus, time from detection of a state (alarm state) in which a temperature of an element in the LCL filter 120 which is detected by the temperature detection unit 140 exceeds a predetermined value to arrival at a temperature damaging the element in the LCL filter 120 is shorter in the case of the reverse connection of the power line of the LCL filter 120 than that in the case of the stop of the cooling fan 130. When the operation of the PWM rectifier 110 continues in the case of the reverse connection of the power line of the LCL filter 120, the element in the LCL filter 120 may be highly probably damaged.
As described above, stopping the operation of the PWM rectifier 110 in the case of the reverse connection of the power line of the LCL filter 120 is to be immediately performed, whereas stopping the same in the case of the stop of the cooling fan 130 is not always to be immediately performed. In other words, when the element in the LCL filter 120 has an abnormal temperature due to the stop of the cooling fan 130, the increase in temperature in this case is not sharp. Thus, during such a short time as to perform a controlled stop of the motor 30, stopping the operation of the PWM rectifier 110 is not to be immediately performed. On the contrary, when the PWM rectifier 110 is immediately stopped, some hindrances (breakage of a workpiece or a tool, and the like) to processing using the motor control apparatus may highly probably occur so that an immediate stop of the PWM rectifier is desirably to be avoided as much as possible.